Breaker Loading

[wirebender]

Just wondering how much load a breaker will actually carry.
I went to check out a job site where I am going to be adding some hi bay 400W Metal halides to an existing installation. There are 3 existing lighting circuits (277V) of which two supply the existing hi bays. 20 amp Cutler Hammer circuit breakers with #12 thhn. One has a load of 25 amps and the other 23.3. Why isn't the breaker opening the circuit?

 

[peter d]

A 20 amp breaker may carry a 23 amp load indefinitely without tripping. Simple molded case circuit breakers just aren't precise enough to trip the instant the current reaches 20 amps.

 

[480sparky]

Couple of days ago I came across a 20-amp breaker that was obviously overloaded. It would audibly hum when I turned it on, and would take 8-10 seconds to trip.

I put my ampmeter on it. It read 59.8 amps.

So 22-23 amps may ver well never trip a 20amp breaker.

 

[mdshunk]

I've measured as high as 27 amps on a 20 amp breaker that wasn't tripping. It was pretty easy to spot on the IR picture, but it was humming along aparently for years that way. A typical breaker tripping is as much a function of the magnitude of the overload as it is the amount of time the overload is occurring. Breakers have "time-current trip curves" plotted on a graph, where one leg of the graph is the current and the other leg is the amount of time. Small overload can exist for a long time, and a big overload can only exist for a short time. A hard short is pretty much immediate. I suspect that this is one of the reasons why the NEC puts so much headroom in the required wire gauge sizes per a certain ampacity.

 

[iwire]

I agree with the others but also will add you meter may not be reading acutely if it is not a true RMS type meter.

 

[drbond24]

I attached a time-current curve for a GE 20 A breaker. Under a load of 40 A (double it's trip rating), it is supposed to trip somewhere between 20 and 80 seconds. You have to be up around 10 to 15 times the trip rating before it just trips instantaneously.

 

[don_resqcapt19]

Take a look at the curve linked to above, this 20 amp breaker might never trip with a load of 27 amps under the conditions specified on the trip curve sheet.
Also are the numbers for the tempature shift index at the top of the curve backwards? It looks like they are saying this breaker will trip sooner in a lower ambient.
Don

 

[wirebender]

I agree with the others but also will add you meter may not be reading acutely if it is not a true RMS type meter.

I only used the meter for verification when i saw they had at least 14 fixtures at 1.7A each on one switch (no contactor).

Does anyone see this as something to bring to the attention of the owners? I know it is a violation but the wire isn't even warm nor is the breaker.

 

[cschmid]

well if you are talking a GE breaker it might not rip at all..I was working on a well pump upgrade and I could not find the breaker so I decided to short it out to trip breaker..it welded the end of the wires off and never tripped the breaker..
I have seen lighting loads exceed breaker limits and hold for along time..I did a lighting job couple yrs ago. the breaker on the light they had in place were tandem 20 amp SQDQO breakers..The load on the breakers were 25 to 27 amps..there were four breakers side by side and the panel had been on fire sometime previous to this as I could see the burn stains on the panel..the insulation on the wires attached to the breakers was all discolored and brittle..there was couple of violations on this panel it took three panels to replace the one it was so over loaded..So breakers are a convient way to protect wiring but for sensitive and exspensive electrical equipment I still use fuses..

 

[ptonsparky]

30.4 amps on a GE 20 amp a couple days ago. Not warm and seem to be just happy with the load. True RMS and comparable to all my other amp clamps.

 

[brian john]

There are alot of variables to consider, Ambient, loading of surrounding circuit breakers, panel fill, load conductor and line bus terminations, did the electrician reinstall the dead front and cover.....

 

[wirebender]

:-? :-? :-? :-?